Thermoelectric conversion material and method of obtaining electrical power using thermoelectric conversion material

ABSTRACT

A thermoelectric conversion material has a composition represented by the chemical formula Li3-aBi1-bSib, in which the range of values a and b is: 0≤a≤0.0001, and −a+0.0003≤b≤0.023; 0.0001≤a&lt;0.0003, and −a+0.0003≤b≤exp[−0.046×(ln(a))2−1.03×ln(a)−9.51]; or 0.0003≤a≤0.085, and 0&lt;b≤exp[−0.046×(ln(a))2−1.03×ln(a)−9.51], and in which the thermoelectric conversion material has a BiF3-type crystal structure and has a p-type polarity.

BACKGROUND 1. Technical Field

The present disclosure relates to a thermoelectric conversion materialand a method of obtaining electrical power using a thermoelectricconversion material.

2. Description of the Related Art

When a temperature difference is provided between two ends of athermoelectric conversion material, an electromotive force that isproportional to the temperature difference is generated. Thisphenomenon, in which heat energy is converted into electrical energy, isknown as the Seebeck effect. In thermoelectric power generationtechnology, heat energy is directly converted into electrical energy byusing the Seebeck effect.

As well known in the technical field of thermoelectric conversionmaterials, the performance of a thermoelectric conversion material usedin a thermoelectric conversion device is evaluated by a figure of meritZT. The ZT is represented by the following formula (I):ZT=S ² σT/κ  (I)where S is the Seebeck coefficient of a material, σ is the electricalconductivity of the material, and κ is the thermal conductivity κ. Thehigher the ZT value, the higher the thermoelectric conversionefficiency.

Non-patent Literature 1 (G.-T. Zhou et al., “Microwave-assistedsolid-state synthesis and characterization of intermetallic compounds ofLi₃Bi and Li₃Sb”, Journal of Materials Chemistry, 13, p. 2607-2611,(2003)) and Non-patent Literature 8 (Sean M. McDeavitt, “Synthesis andcasting of a lithium-bismuth compound for an ion-replacementelectrorefiner”, Light Metals (Warrendale, Pa.), pp. 1139-1142, (1999))each disclose a method of producing a Li₃Bi crystalline substance.

SUMMARY

One non-limiting and exemplary embodiment provides a novelthermoelectric conversion material.

In one general aspect, the techniques disclosed here feature athermoelectric conversion material having a composition represented bythe chemical formula Li_(3-a)Bi_(1-b)Si_(b), in which the thermoelectricconversion material has a BiF₃-type crystal structure and has a p-typepolarity, and in which any one of the following formulas (I) to (III) issatisfied: 0≤a≤0.0001, and −a+0.0003≤b≤0.023 (I); 0.0001≤a<0.0003, and−a+0.0003≤b≤exp[−0.046×(ln(a))²−1.03×ln(a)−9.51] (II); and0.0003≤a≤0.085, and 0<b≤exp[−0.046×(ln(a))²−1.03×ln(a)−9.51] (III).

The present disclosure provides a novel thermoelectric conversionmaterial.

It should be noted that general or specific embodiments may beimplemented as a system, a method, an integrated circuit, a computerprogram, a storage medium, or any selective combination thereof.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a crystal structure of Li₃Bi;

FIG. 2 is a graph showing a diffraction X-ray intensity distribution ofa crystal structure of Li₃Bi; and

FIG. 3 is a graph in which points representing Examples 1 to 10,Comparative Examples 1 to 3, and Reference Examples 1 to 4 are plottedon the a-b plane.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be described below withreference to the drawings.

A thermoelectric conversion material according to the present disclosurehas a composition represented by the following chemical formula (I):Li_(3-a)Bi_(1-b)Si_(b)  (I).

FIG. 1 is a schematic diagram of a crystal structure of Li₃Bi. As isalso disclosed in Non-patent Literature 1, the crystal structure ofLi₃Bi includes a BiF₃-type crystal structure or an AlCu₂Mn-type crystalstructure. Both the BiF₃-type crystal structure and the AlCu₂Mn-typecrystal structure belong to the space group Fm-3m. In Non-patentLiterature 1 and Non-Patent Literature 8, the Li₃Bi crystallinesubstance is not treated as a thermoelectric conversion material.Therefore, Non-patent Literature 1 and Non-Patent Literature 8 do notdisclose a figure of merit ZT.

By using a material search method based on data science, referred to asmaterial informatics, the present inventors calculated predictive valuesof figures of merit ZT on tens of thousands of compounds contained inthe Inorganic Crystal Structure Database. A prediction model for thefigure of merit ZT, which was independently established by the presentinventors, was used in the calculation. This prediction model has higheraccuracy than existing methods. Therefore, by using this predictionmodel, it is possible to obtain prediction results that are morereliable than existing ones. Accordingly, the present inventors studiedwhether or not a Li₃Bi crystalline substance, which had not been treatedas a thermoelectric conversion material, was a promising material as athermoelectric conversion material.

When a Li₃Bi crystalline substance is in a state of not having defects,the Li₃Bi crystalline substance lacks carriers. Therefore, a defectlessLi₃Bi crystalline substance cannot be expected to have a high figure ofmerit ZT. Accordingly, in order to improve the figure of merit ZT, thepresent inventors studied to introduce defects into a Li₃Bi crystallinesubstance to generate p-type carriers. As a result, the presentinventors hit upon two types of materials; a material having defectscaused by substitution of the Li site by vacancies, and a materialhaving defects caused by substitution of the Bi site by the element Si.

The present inventors made calculations and deduced a range of values aand b in which the Li_(3-a)Bi_(1-b)Si_(b) crystalline substance only canbe stably obtained. Furthermore, by calculating figures of merit ZT inthe range, the present inventors found a range of values a and b inwhich a high figure of merit ZT of 0.4 or more can be obtained.Specifically, as verified in Examples 1 to 10, Comparative Examples 1 to3, and Reference Examples 1 to 4, which will be described later, thethermoelectric conversion material has a high figure of merit ZT of 0.4or more when any one of the following formulas (I) to (III) issatisfied:0≤a≤0.0001, and −a+0.0003≤b≤0.023  (I);0.0001≤a<0.0003, and−a+0.0003≤b≤exp[−0.046×(ln(a))²−1.03×ln(a)−9.51]  (II); and0.0003≤a≤0.085, and 0≤b≤exp[−0.046×(ln(a))²−1.03×ln(a)−9.51]  (III).Specifically, in the range, the figure of merit ZT is 0.44 or more.Refer to Table 1.(Production Method)

An example of a method of producing a thermoelectric conversion materialaccording to the present disclosure will be described below on the basisof the disclosures of Non-patent Literature 1 and Non-patent Literature8.

Non-patent Literature 1 discloses a method of producing a Li₃Bicrystalline substance. In the production method disclosed in Non-patentLiterature 1, first, in an argon gas-filled glove box, a Li foil andgranular Bi, at a molar ratio of 43:12, are placed in an aluminacrucible. Then, the Li foil and the granular Bi are pressed togetherinside the crucible. Next, the crucible is placed into a carbon-coatedquartz tube. The inside of the quartz tube is evacuated to a degree ofvacuum of 6.67×10⁻³ Pa. The quartz tube is sealed using an oxygen gasflame. The sealed quartz tube is placed in a microwave cavity and isirradiated with microwave. In this way, a Li₃Bi crystalline substance isobtained. The quartz tube is unsealed in the glove box, and theresulting Li₃Bi crystalline substance is taken out of the quartz tube.

Non-patent Literature 8 also discloses a method of producing a Li₃Bicrystalline substance. In the production method disclosed in Non-patentLiterature 8, first, a tantalum crucible is accommodated in an inductioncasting furnace connected to an inert-gas-filled glove box. Li and Bi,at a molar ratio Li:Bi of 3:1, are introduced into the crucible. Theinside of the furnace is heated slowly. As the inside of the furnace isheated, first, Li is melted. After melting of Li, Bi is melted. Thus, areaction between Li and Bi proceeds to obtain a reaction product. Theresulting reaction product is heated to 1200° C. and homogenized. Thereaction product is cooled, and a Li₃Bi crystalline substance isobtained.

In both of the production methods disclosed in Non-patent Literature 1and Non-patent Literature 8, the deficient amount of Li can becontrolled by changing the amount of Li as a starting material relativeto the amount of Bi.

Furthermore, there is no big difference between the atomic radius of theelement Bi (0.156 nm) and the atomic radius of the element Si (0.111nm). Therefore, it is considered that by adding Si as a startingmaterial, the element Bi can be partially substituted by the element Si.The substitution amount by Si can be controlled by changing the amountof Si as a starting material relative to the amount of Bi.

For the reasons described above, it is considered that theLi_(3-a)Bi_(1-b)Si_(b) crystalline substance according to the presentdisclosure can be produced with reference to the production methoddisclosed in Non-patent Literature 1 or Non-patent Literature 8.

(Method of Obtaining Electrical Power using Thermoelectric ConversionMaterial)

In this embodiment, by applying a temperature difference to theLi_(3-a)Bi_(1-b)Si_(b) crystalline substance according to the presentdisclosure, electrical power can be obtained.

Examples

The thermoelectric conversion material according to the presentdisclosure will be described in more detail with reference to theexamples below.

(Analysis of Crystal Structure)

Non-patent Literature 1 discloses that the crystal structure of a Li₃Bicrystalline substance belongs to a BiF₃-type structure. The X-raydiffraction peaks of the BiF₃-type structure can be confirmed on thebasis of measurement by an X-ray crystal diffraction method. FIG. 2 is agraph showing a diffraction X-ray intensity distribution of a crystalstructure of Li₃Bi obtained, using software (RIETAN, source URL:http://fujioizumi.verse.jp/download/download.html), by calculating thecrystal structure factor F and integrated diffraction intensity I ofLi₃Bi.

The crystal structure factor F was obtained by the following relationalexpression (1):F=Σf _(i) exp(2πir _(i) Δk)  (1)where r_(i) is the position vector of an atom in a crystal, f_(i) is theatomic scattering factor of an atom at a position r_(i), and Δk is thedifference in X-ray wave vector before and after scattering.

The integrated diffraction intensity I was obtained by the followingrelational expression (2):I=I _(e) L|F| ² N ²  (2)where I_(e) is the scattering intensity of an electron, N is the numberof unit lattices in a crystal, and L includes the suction factor and isthe coefficient dependent on experimental conditions.

The thermoelectric conversion material according to the presentdisclosure has defects caused by partial substitution of the element Lisite by vacancies or defects caused by partial substitution of theelement Bi by the element Si in a Li₃Bi crystal. The defects can causelattice deformation in the crystal structure. Therefore, it is expectedthat peaks in a diffraction X-ray intensity distribution of thethermoelectric conversion material according to the present disclosuremay be slightly shifted from the peaks of the Li₃Bi crystallinesubstance shown in FIG. 2 and disclosed in Non-patent Literature 1.

(Stable Composition Range Calculation Method)

The present inventors calculated the band structure of the Li₃Bicrystalline substance by calculations based on the density functionaltheory (hereinafter, referred to as “DFT”). As a result, the presentinventors had conclusion that the Li₃Bi crystalline substance is asemiconductor. In the calculations based on DFT, the present inventorsused the first-principles electronic structure calculation program(Vienna Ab initio Simulation Package, source: https://www.vasp.at/).This program is, hereinafter, referred to as “VASP”.

In order to maximize the figure of merit ZT, it is needed to introducedefects into a semiconductor to generate p-type or n-type carriers. Bygenerating carriers, it is expected that a large electromotive forcewill be obtained when a temperature difference is provided between twoends of a thermoelectric conversion material. It is not obvious whattypes of defects will need to be introduced to obtain a high carrierconcentration. In order to enhance the figure of merit ZT, it is neededto find types of defects that enable a high carrier concentration.

The Li₃Bi crystalline substance disclosed in each of Non-patentLiterature 1 and Non-patent Literature 8 does not have defects and thuslacks carriers. Therefore, the Li₃Bi crystalline substance cannot beexpected to have a high figure of merit ZT. In order to enhance thefigure of merit ZT, the present inventors studied about, for example,defects that might generate vacancies in the crystal structure of Li₃Bi,or substitution by other elements.

As defects for generating p-type carriers, the present inventors studiedabout two types of defects: (I) defects that generate vacancies in thecrystal structure of Li₃Bi, and (II) defects caused by substitution ofthe element Bi by other elements. This study was performed by thepresent inventors on the assumption that, even when the defects wereintroduced into the crystal structure of Li₃Bi, the BiF₃-type structurewas maintained. In other words, in the following calculation targets,the present inventors assumed that each crystal structure had theBiF₃-type structure.

Regarding defects that generate vacancies in the crystal structure ofLi₃Bi, the defect formation energy E_(form) when vacancies are generatedin the Li site and the defect formation energy E_(form) when vacanciesare generated in the Bi site were calculated.

As a result, the present inventors found that the p-type carrierconcentration is high when vacancies are generated in the Li site, whilethe p-type carrier concentration is low when vacancies are generated inthe Bi site. The defect formation energy E_(form) will be described indetail later.

Regarding defects caused by substitution of the element Bi by otherelements, as the candidates for the other elements, the presentinventors tried to use elements Si, Ga, and Pb. The present inventorsselected the elements Si, Ga, and Pb from the viewpoint that each ofthem has a smaller valence than that of the element Bi and that there isno big difference in ionic radius between each of them and the elementBi. The present inventors calculated the defect formation energyE_(form) of the crystal structure of Li₃Bi in which Bi was partiallysubstituted by any one of Si, Ga, and Pb.

As a result, the present inventors found that the p-type carrierconcentration is high when substituted by the element Si, while thep-type carrier concentration is low when substituted by the element Gaor Pb.

On the basis of the above considerations, as the defects that generatep-type carriers, the present inventors hit upon two types: defects thatgenerate vacancies in the Li site, and defects caused by substitution ofthe element Bi by the element Si.

The present inventors calculated the stable composition range ofLi_(3-a)Bi_(1-b)Si_(b) according to the present disclosure. The term“stable composition range” means a composition range in which a singlecrystal phase can be obtained and two or more crystal phases are notformed. Hereinafter, the “single crystal phase” is referred to as the“single phase”. The stable composition range of Li_(3-a)Bi_(1-b)Si_(b)means a composition range in which the Li_(3-a)Bi_(1-b)Si_(b)crystalline substance only can be obtained. Outside this range, inaddition to the Li_(3-a)Bi_(1-b)Si_(b) crystalline substance, crystalphases of other compositions are precipitated, and a single-phaseLi_(3-a)Bi_(1-b)Si_(b) crystalline substance cannot be obtained.

The stable composition range was calculated on the basis of the contentsdisclosed in Non-patent Literature 2 (C. G. Van de Walle et al.,“First-principles calculations for defects and impurities: Applicationsto III-nitrides”, Journal of Applied Physics, 95, p. 3851-3879, (2004))and Non-patent Literature 3 (Y. Koyama et al., “First principles studyof dopant solubility and defect chemistry in LiCoO₂”, Journal ofMaterials Chemistry A, 2, p. 11235-11245, (2014)).

Specifically, first, in Li_(3-a)Bi_(1-b)Si_(b), the range of values aand b in which the BiF₃-type crystal structure was stabilized wascalculated by a method based on the theory of defect formation insemiconductors disclosed in Non-patent Literature 2.

The defect formation energy E_(form) in Li₃Bi was evaluated on the basisof the following relational expression (3):E _(form)(μ_(i) ,q,E _(F))=E _(defect) −E _(pure) −Σn _(i)μ_(i) +q(E_(VBM) +E _(F))  (3)where E_(defect) is the total energy when defect is present, E_(pure) isthe total energy of perfect crystal without defect, n_(i) is the amountof increase or decrease of constituent element i due to defect, μ_(i) isthe chemical potential of element i, q is the charge state of defect,E_(VBM) is the energy of an electron at the valence band maximum ofLi₃Bi which is a semiconductor, and E_(F) is the Fermi energy ofelectron.

These energy values were evaluated by applying DFT calculations within arange of generalized gradient approximation. Note that the E_(form) wascalculated in consideration of the pattern in which vacancies weregenerated in the Li site and the pattern in which Bi was substituted bySi.

By using the defect formation energy E_(form) obtained by the relationalexpression (3), the volume density N_(D) of each defect was evaluated onthe basis of the following relational expression (4) in accordance withthe Boltzmann distribution:N _(D)(μ_(i) ,q,E _(F))=N _(site)×exp[−E _(form)(μ_(i) ,q,E _(F))/k _(b)T]  (4)where N_(site) is the volume density of the site in which the defect inconsideration can be formed, k_(b) is the Boltzmann constant, and T isthe absolute temperature.

The charge-neutral condition in which the total amount of the charge ofeach defect q×N_(D) and the charge Q_(e) of carrier doped into asemiconductor is 0 is always satisfied. On the basis of thecharge-neutral condition, the Fermi energy and the carrier concentrationwere determined in each composition. At that time, the carrierconcentration p of the valence band and the carrier concentration n ofthe conduction band were obtained from the following relationalexpressions (5) to (7):p=1−∫D _(VB)(E)[1−f(E;E _(F))]dE  (5)n=∫D _(CB)(E)f(E;E _(F))dE  (6)f(E;E _(F))=1/[exp((E−E _(F))/K _(B) T)+1]  (7)where D_(VB)(E) and D_(CB)(E) are the electron state density of thevalence band and the electron state density of the conduction band,respectively, obtained by DFT calculations; f(E;E_(F)) is the Fermidistribution function; and T is the absolute temperature at whichthermoelectric conversion characteristics are evaluated. The chargedensity of all carriers Qe was calculated in accordance with the formulaQe=e×(n−p), where e is the charge possessed by one electron.

By using the same method as that of Non-patent Literature 3, the presentinventors calculated the range in which the chemical potential μ_(i) ofeach element was allowed. Furthermore, the range of defect density wascalculated by the relational expression (4). On the assumption that therange in which metal Li, metal Bi, or LiBi is formed is a range in whichthe crystal structure is not stabilized, the chemical potential range inwhich a single-phase Li_(3-a)Bi_(1-b)Si_(b) crystalline substance isobtained is set as a range excluding the range in which the crystalstructure is not stabilized. Within the obtainable chemical potentialrange, the present inventors evaluated the defect density and itsrelated chemical formula composition. The composition was determined asa function of the chemical potential of each element. For example, undercertain chemical potential conditions, in the case where 5% of all Lisites in the Li₃Bi crystal becomes deficient to form vacancies, thecomposition is Li_(2.85)Bi.

On the basis of the method described above, the present inventorsevaluated allowed chemical potential ranges for the Li atom, the Biatom, and the Si atom of Li_(3-a)Bi_(1-b)Si_(b). Within the chemicalpotential ranges, the present inventors evaluated the density of defectsthat generated vacancies in the Li site and the density of defectscaused by substitution of the Bi site by the element Si.

Regarding the Li_(3-a)Bi crystalline substance, the calculation resultsshowed that a single-phase BiF₃-type crystal can be obtained in therange of 0≤a≤0.654.

Regarding the Li_(3-a)Bi_(1-b)Si_(b) crystalline substance, thecalculation results showed that a single-phase BiF₃-type crystal can beobtained in the range of 0≤b≤0.023.

Regarding the Li_(3-a)Bi_(1-b)Si_(b) crystalline substance, ranges inwhich a single-phase BiF₃-type crystal can be obtained in the range of0≤a≤0.085 and 0≤b≤0.023 were calculated. As a result, calculationsshowed that a single-phase BiF₃-type crystal can be obtained in theranges of (i) 0≤a≤0.0001, and 0≤b≤0.023, and (ii) 0.0001≤a≤0.654, and0≤b≤exp[−0.046×(ln(a))²−1.03×ln(a) 9.51].

The formula b=exp[−0.046×(ln(a))²−1.03×ln(a)−9.51] was obtained bycomputing a smooth fitting curve connecting four points: (a, b)=(0.0001,0.023), (0.002, 0.009), (0.007, 0.004), and (0.085, 0.0007). Underchemical equilibrium, two types of defect concentrations a and b arebound by the law of mass action represented by a^(A)b^(B)=equilibriumconstant. The fitting curve in which ln(b) is expressed by the quadraticpolynomial on ln(a), within a narrow composition range, is based on thelaw of mass action, i.e., the presence of linear relationship betweenln(a) and ln(b). Note that these four points correspond to compositionsof solubility limits in the range of a and b in which a single-phaseLi_(3-a)Bi_(1-b)Si_(b) crystalline substance is obtained.

As will be described later, although a single-phaseLi_(3-a)Bi_(1-b)Si_(b) crystalline substance is obtained in the range ofa and b described above, all the substances belonging to the range donot necessarily have a high figure of merit ZT.

(Method for Calculating Figure of Merit of Thermoelectric Conversion)

The thermoelectric conversion efficiency is determined by the figure ofmerit ZT of a material. The ZT is defined by the following relationalexpression (8):ZT=S ² σT/(κ_(e)+κ_(lat))  (8)where S is the Seebeck coefficient, σ is the electrical conductivity, Tis the absolute temperature in the evaluation environment, κ_(e) is theelectron thermal conductivity, and κ_(lat) is the lattice thermalconductivity. Regarding S, σ, and κ_(e), the VASP code and the BoltzTraPcode (refer to Non-patent Literature 4: G. K. H. Madsen et al.,“BoltzTraP. A code for calculating band-structure dependent quantities”,Computer Physics Communications, Volume 175, p. 67-71, (2006)) wereused, and evaluations based on the Boltzmann transport theory were made.The electron relaxation time τ, which is a parameter for determining σ,was calculated by simultaneously solving the following relationalexpression (9) regarding the mobility μ and the theoretical formula (10)disclosed in Non-patent Literature 5 (H. Wang et al., in ThermoelectricNanomaterials, ed, K. Koumoto and T. Mori, Springer, Berlin Heidelberg,vol. 182, ch. 1, p. 3-32, (2013)):μ=eτ/m*  (9)μ=(8π)^(1/2)(h/2π)⁴ eB/3m* ^(5/2)(k _(b) T)^(3/2) g ²  (10)where e is the elementary charge, m* is the effective mass of carrier, Bis the elastic constant, and g is the deformation potential. The valuesof m*, B, and g were calculated by the density functional theory usingthe VASP code. The value of g was calculated by the relationalexpression g=−Δε/(ΔI/I) disclosed in Non-patent Literature 6 (J. Chen etal., “First-Principles Predictions of Thermoelectric Figure of Merit forOrganic Materials: Deformation Potential Approximation”, Journal ofChemical Theory and Computation, 8, p. 3338-3347, (2012)), where Δε isthe amount of change in the energy level of the band edge when thelattice constant I is changed by ΔI.

The lattice thermal conductivity was calculated using the followingempirical formula (11) based on the Debye-Callaway model disclosed inNon-patent Literature 7 (J. Yang et al., “Material descriptors forpredicting thermoelectric performance”, Energy & Environmental Science,8, p. 983-994, (2015)):κ_(L) =A ₁ Mv ³ /V ^(2/3) n ^(1/3) +A ₂ v/V ^(2/3)(1−1/n ^(2/3))  (11)where M is the average atomic mass, v is the longitudinal acoustic wavevelocity, V is the volume per one atom, and n is the number of atoms ina unit cell. In the calculation, the values A₁ and A₂ disclosed inNon-patent Literature 5 were used.(Calculation Results of Figure of Merit)

In the range in which the BiF₃-type crystal structure ofLi_(3-a)Bi_(1-b)Si_(b) is most stable obtained by the method describedabove, the present inventors evaluated thermoelectric conversioncharacteristics. Table 1 shows prediction results of thermoelectricconversion characteristics at 300 K in Examples 1 to 10, ComparativeExamples 1 to 3, and Reference Examples 1 to 4.

TABLE 1 S σ κ a b (μV/K) (S/cm) (W/mK) ZT Reference 0.0003 0 572 15 0.330.44 Example 1 Reference 0.004 0 343 215 0.47 1.61 Example 2 Reference0.016 0 238 783 0.89 1.50 Example 3 Reference 0.085 0 119 4182 3.38 0.57Example 4 Example 1 0 0.0003 556 17.7 0.33 0.50 Example 2 0 0.010 275494 0.68 1.65 Example 3 0 0.023 210 1118 0.82 1.31 Example 4 0.00010.023 210 1117 1.13 1.31 Example 5 0.0002 0.0001 569 15 0.33 0.45Example 6 0.0003 0.0003 369 155 0.43 1.48 Example 7 0.002 0.001 375 1450.42 1.45 Example 8 0.002 0.009 272 511 0.69 1.64 Example 9 0.007 0.004272 506 0.69 1.64 Example 10 0.085 0.0007 119 4182 3.38 0.57 Comparative0 0 670 1.1 0.32 0.05 Example 1 Comparative 0.654 0 34 32206 23.9 0.05Example 2 Comparative 0.0001 0.00005 590 11 0.32 0.36 Example 3

The material of Comparative Example 1 is a Li₃Bi crystalline substancewithout defect.

The materials of Reference Examples 1 to 4 and Comparative Example 2 areLi_(3-a)Bi crystalline substances which are made deficient of Li.

The materials of Examples 1 to 3 are Li_(3-a)Bi_(1-b)Si_(b) crystallinesubstances in which Bi is substituted by Si.

The materials of Examples 4 to 10 and Comparative Example 3 areLi_(3-a)Bi_(1-b)Si_(b) crystalline substances which are made deficientof Li and in which Bi is substituted by Si.

As shown in Table 1, in Reference Examples 1 to 4, in the compositionrange of 0.0003≤a≤0.085, p-type characteristics with a positive S valuewere obtained, and a high ZT of 0.44 or more, greatly higher than thatof Comparative Examples 1 to 3, was obtained.

In Examples 1 to 3, in the composition range of 0.0003≤b≤0.023, p-typecharacteristics with a positive S value were obtained, and a high ZT of0.50 or more, greatly higher than that of Comparative Examples 1 to 3,was obtained.

In Examples 4 to 10, under the compositional conditions of 0.0001 a0.085, and 0.0001≤b≤0.023, p-type characteristics with a positive Svalue were obtained, and a high ZT of 0.45 or more, greatly higher thanthat of Comparative Examples 1 to 3, was obtained.

The ZT values of the materials of Reference Examples 1 to 4 and Examples1 to 10 were each higher than 0.4 at 300 K. Therefore, the presentinventors consider that the thermoelectric conversion materialsaccording to the present disclosure are useful in thermoelectric powergeneration in the low-temperature range of 200° C. or lower.

FIG. 3 is a graph in which points representing Examples 1 to 10,Comparative Examples 1 to 3, and Reference Examples 1 to 4 are plottedon the a-b plane.

The curve passing through the four points representing Examples 4, 8, 9,and 10 shown in FIG. 3 (i.e., (a, b)=(0.0001, 0.023), (0.002, 0.009),(0.007, 0.004), and (0.085, 0.0007)) is represented by the formula:b=exp[−0.046×(ln(a))²−1.03×ln(a)−9.51].

The straight line passing through the three points representingReference Example 1, Example 1, and Example 5 (i.e., (a, b)=(0.0003, 0),(0, 0.0003), and (0.0002, 0.0001)) is represented by the formula:b=−a+0.0003.

The shaded area shown in FIG. 3 is a region surrounded by the followingthree formulas (I), (II), and (III): 0≤a≤0.0001, and −a+0.0003≤b≤0.023(I); 0.0001≤a≤0.0003, and−a+0.0003≤b≤exp[−0.046×(ln(a))²−1.03×ln(a)−9.51] (H); and0.0003≤a≤0.085, and 0≤b≤exp[−0.046×(ln(a))²−1.03×ln(a) 9.51] (III).

The thermoelectric conversion material according to the presentdisclosure can be used in a thermoelectric conversion device whichconverts heat energy into electrical energy.

What is claimed is:
 1. A thermoelectric conversion material comprising acomposition represented by the chemical formula Li_(3-a)Bi_(1-b)Si_(b),wherein: the thermoelectric conversion material has a crystal structuresame as BiF₃, and has a p-type polarity, and any one of the followingformulas (I) to (III) is satisfied:0≤a≤0.0001, and −a+0.0003≤b≤0.023  (I);0.0001≤a<0.0003, and −a+0.0003≤b≤exp[−0.046×(ln(a))²−1.03×ln(a)−9.51]  (II); and0.0003≤a≤0.085, and 0≤b≤exp[−0.046×(ln(a))²−1.03×ln(a)−9.51]  (III). 2.The thermoelectric conversion material according to claim 1, whereina=0, and 0.0003≤b≤0.023.
 3. A method of obtaining electrical power usinga thermoelectric conversion material, the method comprising: applying atemperature difference to the thermoelectric conversion material togenerate the electrical power, wherein: the thermoelectric conversionmaterial has a composition represented by the chemical formulaLi_(3-a)Bi_(1-b)Si_(b), has a crystal structure same as BiF₃, and has ap-type polarity, and any one of the following formulas (I) to (III) issatisfied:0≤a≤0.0001, and −a+0.0003≤b≤0.023  (I);0.0001≤a<0.0003, and −a+0.0003≤b≤exp[−0.046×(ln(a))²−1.03×ln(a)−9.51]  (II); and0.0003≤a≤0.085, and 0≤b≤exp[−0.046×(ln(a))²−1.03×ln(a)−9.51]  (III).